In the hydroformylation of terpenes under the conditions customary in the oxo reaction and in the presence of dicobalt octacarbonyl, complex product mixtures are formed which contain aldehydes, alcohols, acetals, ethers, and rearrangement products of the respective terpene hydrocarbons (W. H. Clement et al., Ind. Eng. Chem. Prod. Res. Dev., Vol. 4 (1965), pp. 283-286). Less complicated compound mixtures have been obtained with the hydroformylation of .alpha.-pinene in the presence of rhodium catalysts (W. Himmele et al., Tetrahydron Letters, 1976, pp. 907-910). The reaction mixture contained in this case, in addition to the desired 3-formylpinane, a considerable proportion of the two isomeric 10-formylpinanes, which can form only if under the reaction conditions an isomerization of the double bond of the .alpha.-pinene occurs.